U.S. Pat. No. 3,756,782 discloses a method and apparatus for determining the carbon dioxide content of a blood sample in which an acid-containing syringe is coupled in gas-tight relation to a vial containing a blood sample. The acid is injected into the vial and the sample, upon such acidification, releases carbon dioxide which may be quantitatively measured by the extent of displacement of the syringe plunger. As brought out in that patent, such a system is based on the classical Van Slyke procedure but, unlike prior systems used in the clinical laboratory, is relatively uncomplicated in structure and operation. The system of U.S. Pat. No. 3,756,782 does have a shortcoming; however, it is not well suited for determining the carbon dioxide content of micro samples (i.e., under 0.5 milliliters) of biological fluid.
Equipment for measuring the carbon dioxide content in micro samples is known, as disclosed, for example, in U.S. Pat. No. 2,680,060. The disadvantages of available micro-gasometers have already been indicated; in general, they are bulky, expensive, and complex in both structure and operation. It will be readily appreciated that operative complexities are particularly undesirable, not simply because they consume time and cause delays, but because they increase the risk of manipulative error which, in any diagnostic procedure, may have most serious consequences. Also, because such devices are sometimes provided with mercury-containing tubes to measure pressure, they present additional risks which attend the use of a hazardous substance.